https://leetcode.cn/problems/reverse-linked-list/description/
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head==nullptr||head->next==nullptr)
            return head;
        
        ListNode*  newnode=reverseList(head->next);
        head->next->next=head;
        head->next=nullptr;
        return newnode;
    }
};